CHARACTERIZATION OF MITOCHONDRIAL SMALL-SUBUNIT RIBOSOMAL-RNAS FROM HOLOPARASITIC PLANTS

Mitochondrial small-subunit (19S) rDNA sequences were obtained from 10 angiosperms to further characterize sequence divergence levels and st ructural variation in this molecule. These sequences were derived from seven holoparasitic (nonphotosynthetic) angiosperms as well as three photosynthetic plants. 19S rRNA is composed of a conservative core reg ion (ca. 1450 nucleotides) as well as two variable regions (VI and V7) . In pairwise comparisons of photosynthetic angiosperms to Glycine, th e core 19S rDNA sequences differed by less than 1.4%, thus supporting the observation that variation in mitochondrial rDNA is 3-4 times lowe r than seen in protein coding and rDNA genes of other subcellular orga nelles. Sequences representing four distinct lineages of nonasterid ho loparasites showed significantly increased numbers of substitutions in their core 19S rDNA sequences (2.3-7.6%), thus paralleling previous f indings that showed accelerated rates in nuclear (18S) and plastid (16 S) rDNA from the same plants. Relative rate tests confirmed the accele rated nucleotide substitution rates in the holoparasites whereas rates in nonparasitic plants were not significantly increased. Among compar isons of both parasitic and nonparasitic plants, transversions outnumb ered transitions, in many cases more than two to one. The core 19S rRN A is conserved in sequence and structure among all nonparasitic angios perms whereas 19S rRNA from members of holoparasitic Balanophoraceae h ave unique extensions to the V5 and V6 variable domains. Substitution and insertion/deletion mutations characterized the V1 and V7 regions o f the nonasterid holoparasites. The V7 sequence of one holoparasite (S cybalium) contained repeat motifs. The cause of substitution rate incr eases in the holoparasites does not appear to be a result of RNA editi ng, hence the underlying molecular mechanism remains to be fully docum ented.